Jefferson County is a county on the northern border of the U.S. state of New York. As of the 2010 census, the population was 116,229. Its county seat is Watertown. The county is named after Thomas Jefferson, third President of the United States of America. It is adjacent to Lake Ontario, southeast from the Canada–US border of Ontario.
According to the U.S. Census Bureau, the county has an area of 1,857 square miles (4,810 km), of which 1,269 square miles (3,290 km) is land and 589 square miles (1,530 km) (32%) is water. It is the fourth-largest county in New York by area.
Jefferson County is in New York State’s northern lobe, adjacent to the area where the Saint Lawrence River exits Lake Ontario. It is northeast of Syracuse, and northwest of Utica. The county is at the international border with Canada.
The Black River, which empties into Lake Ontario, is an important waterway in the county. Part of the Tug Hill Plateau is in the southern part of the county. The county contains part of the Thousand Islands in the St. Lawrence River, including such large islands as Carleton Island, Grindstone Island, and Wellesley Island.
As of the census of 2000, there were 111,738 people, 40,068 households, and 28,127 families residing in the county. The population density was 88 people per square mile (34/km²). There were 54,070 housing units at an average density of 42 per square mile (16/km²). The county’s racial makeup was 88.71% White, 5.83% Black or African American, 0.53% Native American, 0.92% Asian, 0.14% Pacific Islander, 2.05% from other races, and 1.82% from two or more races. 4.19% of the population were Hispanic or Latino of any race. 93.2% spoke English and 3.5% Spanish as their first language.
21.9% were of English, 14.1% Irish, 12.8% German, 8.5% French and 8.5% Italian ancestry according to the 2010 American Community Survey.
There were 40,068 households, of which 37.20% had children under the age of 18 living with them, 55.60% were married couples living together, 10.40% had a female householder with no husband present, and 29.80% were non-families. 24.40% of all households were made up of individuals and 10.10% had someone living alone who was 65 years of age or older. The average household size was 2.58 and the average family size was 3.07.
26.50% of the county’s population was under age 18, 11.80% was from age 18 to 24, 31.30% was from age 25 to 44, 19.10% was from age 45 to 64, and 11.30% were age 65 or older. The median age was 32 years. For every 100 females there were 107.30 males. For every 100 females age 18 and over, there were 108.50 males.
The county’s median household income was $34,006, and the median family income was $39,296. Males had a median income of $28,727 versus $21,787 for females. The county’s per capita income was $16,202. About 10.00% of families and 13.30% of the population were below the poverty line, including 16.80% of those under age 18 and 9.20% of those age 65 or over.